A communication system, in general, provides for voice and perhaps data communication between two or more parties. Generally speaking, every communication system must have a way for subscribers to access the system, including the equipment used by the subscribers themselves, and a core network for transporting voice and data traffic from one access location to another. Some type of signaling system must also be in place to facilitate setting up and ending calls, and for the provision of other call-related services.
In a standard wireline system, such as a PSTN (public switched telephone system) for example, subscribers use telephones are connected with stationary access points (for example, a phone jack) in a business or residence. The access point is connected, perhaps through intermediary devices, with a telephone company switching office. The switching office provides access to a hierarchically-arranged network of variously-sized lines and trunks, interconnected with switching and other equipment, that route each call to its destination. When the caller initiates a call by picking up the telephone and dialing a number, a circuit is established through the network to the called party. The circuit remains dedicated to the call until it is completed, then the network resources used for the session are released for use on other calls.
In a mobile communication system, such a PLMN (public land mobile network), subscribers may and frequently do move from one geographic location to another. Instead of a telephone plugged into a jack, a subscriber wishing to make a call uses a MS (mobile station) with a relatively low-power radio transceiver to communicate over an air interface a nearby antenna. There are typically a large number of such antennas distributed over the network coverage area. Each of the antennas is connected with a core network for the routing and transmission of calls. A signaling system is also present in a PLMN so that calls may be set up and ended properly. The signaling system is also used to allow a mobile subscriber to switch from communicating with one antenna to another to allow for relocation even while a call is in progress.
Mobile communication systems are constructed and operating according to a set of standards and protocols. One type system is referred to as GSM (Global System for Mobile communication) and is used extensively throughout the world. Selected components of a typical GSM PLMN 10 are illustrated in FIG. 1. FIG. 1 is presented for the purpose of introducing various network components and will be described only briefly. MSs (mobile stations) access the PLMN 10 through a nearby antenna over an air interface. In the example of FIG. 1, there is shown three BSSs (base stations systems referred to as 16, 17, and 18. BSS 16 includes BSC (base station controller) 41, which is in communication with BTSs (base transceiver stations) 42 and 43. The BTSs include the actual antenna for communication and, as an example, MS 12 is depicted as communicating with BTS 42. Similarly, BSS 17 includes BSC 31, which communicates with BTS 32 and BTS 33, and BSS 18 includes BCS 35 and BTS 36. Here, BTS 36 is shown in communication with MS 14.
Communication between MS 12 and MS 14 is arranged when their respective BSS contacts an MSC with which they are in communication. As shown here, BSS 16 is in communication with MSC 20, and BSS 18 communicates with MSC 30. Through MSC 20 and MSC 30, the call between the two MSs can be arranged. During setup, the MSCs may consult the HLR 15, which tracks the identity and location of MS that belong to PLMN 10. Each MS registers periodically with a nearby BTS and their location may be reported to HLR 15 at that time. An MS from another PLMN may register as well, with their current location stored in a VLR (not shown) associated with each MSC, and reported to the HLR of their PLMN (also not shown).
Although only two MS are shown in FIG. 1, there are typically a large number. At times, the capacity of the network may be reached. In that event, access to the network must be limited. This may be done through admission control. For example, in the PLMN 10 of FIG. 1, voice transport in the interface between a BSC and its respective MSC (referred to sometimes as the “A interface”) uses TDM (time division multiplexing). In TDM each transported frame has a number of time slots, and each time slot may be assigned to a particular call so that many calls may be handled by the same transmission channel. When there are no time slots left to assign, an incoming call request must be rejected.
It is becoming frequently common, however, for communication networks to utilize a packet-switched network, usually operable according to the IP (Internet protocol), to transport voice and other signals. A packet-switched network does not assign time slots, but rather breaks up transmissions into a number of packets of information, each of which is provided with a destination address and routed through the network. IP networks include a large number of routers and similar devices, but do not establish a fixed path for each transmission. Rather, each packet is routed individually, and the several packets of a given transmission may each take different routes through the network. An identifier associated with each packet enables the destination device to reassemble them to form the original transmission. Individual packets are sometimes lost, due to network congestion or equipment malfunction, and lost packets are sometimes re-sent upon request. In general, the small amount of information lost with each packet, which is sometimes recovered through retransmission, does not always affect the transported voice content significantly.
If too many packets are lost, however, the quality of the transmission may degrade. If the loss is attributable to network congestion, numerous requests for re-sending packets compounds the problem. As should be apparent, some form of admission control for such a network would be desirable. Since time slots or circuits are not assigned for the IP portion of the network (sometimes referred to as the IP backbone), however, traditional methods of admission control cannot be used.
This need is exacerbated where the A interface extends across the packet-switched network, since access control in this configuration cannot be imposed in the traditional fashion. The A interface may be extended in this fashion so that only one MGw is required for a call being carried across the network (see FIG. 2). Using only a single MGw, generally speaking, increases the capacity of the network without a large capital expenditure. The IP network may still become congested, however, with an accompanying degradation in quality. There is a need, therefore, for an admission control solution for use in communication networks that utilize an IP backbone.